The present invention relates to disc refiners, and more particularly, to disc refiners of a type that have multiple refining zones within a single refining casing.
U.S. Pat. No. 2,864,562, issued on Dec. 16, 1958 to L. E. Eberhardt et al and entitled "Plural Stage Disc Mill With Back Pressure Control Means For Each Stage", discloses a conventional technique for obtaining two refining "passes" through a single refiner, by first passing the feed stock through a refining zone on the front face of the disc, extracting the partially refined material out of the casing and then reintroducing the partially refined material at the entrance of a second refining zone at the backside of the disc.
Current theory indicates that refining intensity, i.e., the specific refining power or energy per impact of the fiber in the refining zone, is a dominant factor for optimizing and controlling the quality of fiber produced in the refining process. While the refining intensity of various refiner concepts presently available varies over a wide range, the intensity range that any given refiner can operate through is extremely limited. Therefore, pulp properties cannot be significantly modified after the basic refiner type has been selected. Furthermore, the range of practical refining intensities is restricted as a result of material feeding and steam removal limitations. Steam removal is a significant consideration, because, particularly in modern, high speed, high consistency refiners, considerable steam is generated within the refining zone as a result of the heat and friction experienced by the material during refining. The steam itself must be removed, and the influence of steam pressure within the casing must be accommodated.